Raman scattering is an inelastic scattering of a photon which creates or annihilates an optical phonon. Raman scattering is the result of the interaction of incident photons with chemical molecular vibrations (i.e. phonons). A unique molecular structure results in a unique Raman scattering spectrum. Therefore, Raman scattering provides spectral fingerprint details about the molecules, and can also be used to distinguish molecular isomers and chiral molecules from each other.
Raman spectroscopy became commercially available after the invention of lasers in the late 1960's. A laser beam having a narrow line width is used to illuminate the testing chemicals in solid, liquid, or gas forms. The narrow line width of the laser beam can eliminate the overlaps of scattering peaks from photons with various wavelengths. The scattered light is collected by a photon detector such as Charge-Coupled Devices (CCD) or CMOS detector, which produces a Raman spectrum which is the intensity of the scattered light as a function of Raman shift defined as the wavenumber difference between the scattering light and incident laser. The spectral peaks in a Raman spectrum correspond to the vibration strengths of various molecular bonds, thus provide spectral fingerprints of the molecules.
Although Raman scattering is a useful analytical tool, Raman scattering has not been widely applied because of its weak scattering signals. Raman scattering has very weak signals due to the very small scattering cross section of molecules. Typically, only about 10−8 of the incident photons participates in Raman scattering. High power laser and high sensitivity CCD detector can be used to improve the scattering signals; however, it only makes limited improvement in detecting Raman scattered signal, and it requires extra costs and additional hardware, and can cause unexpected damages to samples.
Raman scattering signal can be enhanced by placing testing molecules in the vicinity of roughened surfaces. In Nano-Enhanced Raman Spectroscopy (NERS), or surface-enhanced Raman spectroscopy (SERS), a sample surface can be formed by deposition of metallic particles or clusters. The nano-enhanced Raman scattering phenomena can be explained by interaction between photons with localized electromagnetic field enhancement and chemical enhancement. The enhancement by SERS has been observed in different research labs. An Intel team used a porous silicon structure with coatings of noble metals such as silver on the surface, and demonstrated that Raman scattering signal increased as the porous silicon pore-size was decreased.
Accordingly, there is a need for low cost, convenient SERS devices for a wide-range of commercial applications.